The invention relates to power conversion electronics.
Alternating current (AC) has been almost universally adopted for utility power distribution and consequently is the standard form of electrical power for industrial, commercial, and domestic purposes. Independent of the source of energy used to generate the electricity (for example, hydroelectric, nuclear, solar, wind-power), AC must be provided at a fixed frequency of 60 Hz in North America (usually 50 Hz elsewhere) and phase-synchronized before being made available to the large power grid from which users obtain electricity.
Most utility electrical power is generated and distributed as three-phase power for a number of reasons. Unlike single-phase power, the instantaneous power of three phase (or any balanced polyphase system) is constant. Moreover, the output voltage of a polyphase rectifier is smoother than that of single-phase rectifiers when no filter is used with either. The utilization of transformers and other associated equipment is also improved in polyphase circuits over those with single-phase circuits, and generally results in improved power factor and reduced harmonic distortion.
Semiconductor-based power electronic converters are often used to conform the electrical power generated from various power sources to the 60 Hz fixed frequency, phase-synchronized AC required by the grid. These power converters generally rectify variable-frequency, variable-phase AC power to DC and then convert (invert) the DC back to AC at 60 Hz. Such, rectifier/inverter circuits are also widely used in adjustable speed drives (ASDs) for electric motors, and a number of different design topologies are in use, all of which require an energy storage element to link the converter to the inverter. Unfortunately, the AC-DC-AC rectification/inversion process wastes a portion of the generated power due primarily to the dissipation occurring within the large energy storage devices (e.g., inductors) and within the semiconductor devices themselves.
It is desired that any losses associated with the conversion and regulation of the high power generated in such utility systems be minimized. Synchronous rectifiers may be used in such systems as a means for reducing the losses in rectifying AC to DC. The general concept of synchronous rectification entails the substitution of a low resistance semiconductor switch for each conventional diode element in any one of a number of known rectification circuit topologies. In large industrial power applications, this may potentially result in savings of tens to hundreds of kilowatts of dissipated power over conventional diode rectifier arrangements. Conventional high power diode rectifiers exhibit a slow recovery time when commutating from a conducting state to a reverse blocking state. This slow recovery time results in a brief interphase short circuit condition between individual rectifiers during the diode transition times. The result is a momentary reduction in interphase voltage. This phenomenon is known as "line notching" and requires additional filtering to reduce harmonic pollution of the utility lines. It is important in the application of synchronous rectification that the semiconductor switch commutation intervals not differ from the natural commutation intervals of the conventional diode rectifier circuit topology chosen. That is, forced commutation is generally not permissible as it results in an interphase short circuit.
In single-phase synchronous rectification, the control strategy for the switch operation is derived very simply by a comparison operation which selects one or another switch based on the instantaneous polarity with respect to a common reference of the single-phase voltage to be rectified. In switchmode power supplies operating for example at low voltage and low power levels, the control and gate drives for the switches are identical and are typically derived from the output winding of the transformer providing the source of voltage to be rectified.
On the other hand, with polyphase rectification, the commutation intervals are considerably more complex and cannot be determined on the basis of polarity of the input voltage. Instead, the rectifier commutation points occur at intervals determined by the transitions at which a particular phase voltage becomes most positive or most negative with respect to the other input phases. The complexity increases when considering, for example, 48 pulse (24 phase) rectification which is commonly employed to reduce output ripple in the rectified DC voltage.